Plastic dental compositions are increasingly being used for dental restoration in the dental sector. Such plastic dental compositions usually include a matrix of organic resins and various inorganic fillers. Inorganic fillers predominantly comprise powders of glasses, (glass-) ceramics, quartz or other crystalline substances (e.g. YbF3), sol-gel materials or aerosils which are added to the plastic composition as filling material.
The use of plastic dental compositions is intended to avoid possible harmful side-effects of amalgam and to achieve an improved aesthetic impression. Depending on the plastic dental compositions selected, they can be used for different dental restoration purposes, for example, for tooth fillings as well as for securing parts, such as crowns, bridges and inlays, onlays etc.
The filling material per se is intended to minimize the shrinkage caused by polymerization of the resin matrix during curing. For example, if there is a strong adhesion between the tooth wall and filling, excessive polymerization shrinkage can lead to the tooth wall breaking If the adhesion is inadequate, excessive polymerization shrinkage may result in the formation of peripheral gaps between the tooth wall and filling which can promote secondary caries. Furthermore, certain physical and chemical demands are imposed on the fillers.
It is desirable to process the filling material to form powders that are as fine as possible. The finer the powder, the more homogenous the appearance of the filling. At the same time, the polishing properties of the filling are improved, which in addition to reducing the surface area available for attack also leads to improved resistance to abrasion and therefore to a longer-lasting filling. To enable the powders to be processed successfully, it is also desirable for the powders not to agglomerate. This undesirable effect tends to occur with filling materials produced using sol-gel processes.
Furthermore, it is advantageous if filler particles are coated or at least partially coated with functionalized silane, since this facilitates formulation of dental compositions and improves the mechanical properties.
Furthermore, the refractive index and color of the entire plastic dental composition, including fillers, should be as well-matched as possible to the natural tooth material, so that it is as indistinguishable as possible from the surrounding, healthy tooth material. The grain size of the pulverized filler being as small as possible also helps to achieve this aesthetic criterion.
It is also important for the thermal expansion of the plastic dental composition in the typical range of use, i.e. usually between −30° C. and +70° C., to be matched to that of the natural tooth material in order to ensure that dental restoration measures are sufficiently able to withstand temperature changes. Excessively high stresses caused by temperature changes also can cause the formation of gaps between plastic dental compositions and the surrounding tooth material, which in turn can form sites of attack for secondary caries. In general, fillers with the lowest possible coefficient of thermal expansion are used to compensate for the high thermal expansion of the resin matrix.
Good chemical resistance of the fillers with respect to acids, alkalis and water and good mechanical stability under load, such as, for example, during movement produced by chewing, can also contribute to a long service life for dental restoration measures.
Furthermore, for the treatment of patients, it is imperative that dental restoration measures can be seen in an X-ray image. Since the resin matrix itself is generally invisible in an X-ray image, the fillers must provide the required X-ray absorption. A filler of this type which provides sufficient absorption of X-radiation is described as X-ray opaque. Constituents of fillers, for example, certain components of a glass, or other substances, are generally responsible for X-ray opacity. Such substances are often referred to as X-ray opacifiers. A standard X-ray opacifier is YbF3, which can be added to the filler in crystalline, milled form.
According to International Standard DIN ISO 4049, the X-ray opacity of dental glasses or materials is quoted in relation to the X-ray absorption of aluminum as aluminum equivalent thickness (ALET). The ALET is the thickness of an aluminum sample which has the same absorption as a 2 mm-thick sample of the material to be tested. An ALET of 200% therefore means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 4 mm. Analogously, an ALET of 500% means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 10 mm.
Because plastic dental compositions in use are usually introduced into cavities from cartridges and then modeled in the cavities, such compositions should be at least somewhat thixotropic in the uncured state. This means that viscosity decreases when pressure is exerted, while it is dimensionally stable without the action of pressure.
Among plastic dental compositions, a distinction also should be drawn between dental cements and composites. In the case of dental cements, also known as glass ionomer cements, the chemical reaction of fillers with the resin matrix leads to curing of the dental composition, and consequently the curing properties of the dental composition. Thus, their workability is influenced by the reactivity of the fillers. This often involves a setting process which is preceded by a radical surface curing, for example under the action of UV light. Composites, also referred to as filling composites, contain, by contrast, fillers which are as chemically inert as possible, since their curing properties are determined by constituents of the resin matrix itself and a chemical reaction of the fillers often disrupts this.
Because glasses, due to their different compositions, represent a class of materials with a wide range of properties, they are often used as fillers for plastic dental compositions. Other applications as dental material, either in pure form or as a component of a material mixture, are also possible, for example for inlays, onlays, facing material for crowns and bridges, material for artificial teeth or other material for prosthetic, preservative and/or preventive dental treatment. Glasses of this type used as dental material are generally referred to as dental glasses.
In addition to the dental glass properties described above, it is also desirable for this glass to be free from barium and/or barium oxide (BaO), which are classified as harmful to health, and also from lead and/or lead oxide (PbO) and from other barium and lead compounds.
In addition, it is also desirable for a component of dental glasses to be zirconium oxide (ZrO2). ZrO2 is a widely-used material in the technical fields of dentistry and optics. ZrO2 is readily biocompatible and is distinguished by its insensitivity to temperature fluctuations. It is used in a wide variety of dental supplies in the form of crowns, bridges, inlays, attachment work and implants.
Dental glasses therefore represent glasses of particularly high quality. Glasses of this type also can be used in optical applications, particularly if such applications benefit from the X-ray opacity of the glass. Since X-ray opacity means that the glass absorbs electromagnetic radiation in the region of the X-ray spectrum, corresponding glasses simultaneously act as filters for X-radiation. Sensitive electronic components can be damaged by X-radiation. In the case of electronic image sensors, for example, the passage of an X-ray quantum may damage the corresponding region of the sensor or result in an undesirable sensor signal which can be perceived, for example, as an image disturbance and/or disturbing pixels. For specific applications it is therefore necessary, or at least advantageous, to protect electronic components against X- radiation by using corresponding glasses to filter said components out from the spectrum of the incident radiation.
A number of dental and optical glasses are described in the prior art. Each of these glasses, however, has significant disadvantages, inter alia, in production and/or application. In particular, many such glasses have a relatively large content of fluoride and/or Li2O, which evaporates very readily during the (initial) melting operation, making it difficult to accurately set glass compositions.
For example, DE 60315684 T2 describes a glass filler material for epoxy systems and the production thereof. The desired particles have a particle size of 0.1 μm up to 20 μm and comprise an inner and an outer zone which have different alkali metal concentrations and in which the alkali metal ions of the inner layer do not migrate into the outer layer during the period of use. The depletion of the outer layer takes place in a further step, after the melted glass has been milled, by adding an organic or inorganic acid which is subsequently washed out again. According to the invention, the glass powder produced in this way has a refractive index (nd) of 1.49 to 1.55. In order that the alkali metal ions can be leached out, the molten glass has to have a low chemical resistance.
JP 62012633 describes an ion-exchangeable glass for products having a graded refractive index. In sharp contrast to glasses according to the present invention, however, the glass described in this reference must have a high ZnO content. Such a glass system does not have sufficiently high X-ray opacity.
U.S. 2003/050173 A1 describes a glass substrate for interference filters having a relatively high coefficient of thermal expansion. This adapted coefficient of thermal expansion means that SiO2 is limited to at most 66 mol %. SiO2 acts as a network former and brings about a reduction in the coefficient of expansion. However, glasses with a small SiO2 content generally have low chemical resistances, and thus cannot be used as dental glasses, for example.
JP 2007-290899 A describes technical radiation-shielding glasses which have a small SiO2 content and require the presence of fluorides such as AlF3 or LaF3. During the melting of such glasses, however, fluorides tend to evaporate readily, which makes it difficult to accurately set such glass compositions, resulting in a lack of homogeneity.
U.S. Pat. Nos. 5,976,999 and 5,827,790 relate to glass-like ceramic compositions used, inter alia, for dental porcelain. These references state that CaO and LiO2 must be present in amounts of at least 0.5% by weight and 0.1% by weight, respectively. In addition to the two main additional components from the group consisting of ZrO2, SnO2 and TiO2, a content of CaO therein of at least 0.5% by weight appears to be essential. These components bring about an increased refractive index nd and only partial X-ray opacity. The glasses described in these documents also must contain at least 10% by weight B2O3. The high B2O3 content in combination with the alkali metal content of at least 5% by weight or at least 10% by weight results in unacceptably impaired chemical resistance of such glasses rendering them unsuitable for dental glasses.
Certain chemically inert dental glasses for use as filler in composites are described in DE 198 49 388 A1. The glasses proposed therein must contain significant amounts of ZnO and F which can lead to reactions with the resin matrix, which can in turn have effects on their polymerization properties. In addition, the SiO2 content is limited to 20-45% by weight so that such glasses can contain sufficient X-ray opacifier and F.
W02005/060921 A1 describes a glass filler which, in particular, is intended to be suitable for dental composites. However, this glass filler must contain only 0.05 to 4 mol % alkali metal oxides. This low alkali metal oxide content in combination with metal oxides, in particular in combination with ZrO2, makes such dental glass more inclined to segregate. The segregated regions act as centers for scattering light that passes through, analogous to the Tyndall effect. This may create unfavorable consequences for the optical properties of the dental glass and the aesthetics of the plastic dental compositions produced with segregated dental glasses therefore cannot satisfy relatively high demands.
EP 0885606 B1 describes an alkali metal silicate glass which serves as filling material for dental material. The limited B2O3 content of 0.2 to 10% by weight described in this reference makes it difficult to melt glass with a high SiO2 content, making it expensive and uneconomical to produce such glass. Furthermore, such glasses must contain fluorine. During the melting of such glass, however, fluorides readily evaporate, making it difficult to accurately set the glass composition, leading to a lack of homogeneity. In addition, the content of the component CaO, which imparts X-ray opacity to the glass, is too low, at 0.5 to 3% by weight, to achieve the required X-ray opacity with an ALET of at least 500%. Further components, which ensure the X-ray opacity of the glass, are not present.
DE 4443173 A1 describes a glass which has a high zirconium content, has a ZrO2 content of more than 12% by weight and contains other oxides. Fillers such as these are too reactive, especially for the most modern epoxy-based dental compositions in which excessively rapid, uncontrolled curing may occur. Zirconium oxide in this amount tends to become devitrified. This brings about phase segregation, possibly with nucleation and subsequent crystallization. In addition to ZrO2, the glass described this reference contains no further components which could provide a high X-ray opacity with an ALET of at least 500% (as in glasses according to the present invention). Even if a maximum amount of 30% by weight ZrO2 were present, an X-ray opacity of at least 500% cannot be achieved in such glass system.
Features common to the glasses mentioned in the prior art are that they either (1) have a relatively high refractive index nd, and/or (2) have low weathering resistance and/or (3) are not X-ray opaque and/or (4) are difficult or expensive to produce, and/or (5) contain components which are harmful to the environment and/or to health.